I think it depends on the exchanger site. Because I'm always leaving my altcoins in c-cex but it never lost. It would be better if you will leave your bitcoins or altcoins in some legit exchangers like c-cex and yobit to be sure that your bitcoin or altcoin would never be lost or get stolen maybe.
Is c-cex or yobit more legit than poloniex?
You have got to be joking right? Them two exchanges are scam exchanges and poloniex is the most trusted alt exchange.
I definitely agree that Poloniex is the most solid and respected altcoin exchange in the market, but that doesn't mean other exchanges are scams. Yobit has a horrible support service, which might even take much longer to reply or solve open tickets compared to other exchanges, but that doesn't make it a scam. I can't really comment on C-Cex as I simply don't have much information about this exchange, but from the looks, just as is the case with Yobit, there are a whole bunch of shit coins listed for the sole purpose of earning money. Every fake dev and its shill supporters are looking to buy themselves a position on various exchanges to dump their worthless coins on rookies.